Future base station transmitters will have to process an increasing number of carriers or sub-carriers, e.g. for GSM (Global System for Mobile communication), UMTS (Universal Mobile Telecommunication System) or WiMAX (Worldwide Interoperability for Microwave Access). This leads to an increasing signal bandwidth to be handled. A multi-carrier transmitter comprises an input for receiving digital signals on multiple different channels. The signals on the different channels are to be transmitted on different carrier frequencies. For modulating the signals to their corresponding carrier frequencies, a modulation technique called IQ modulation is known. For IQ modulation the digital signal to be modulated is split into a in-phase (I) component and a quadrature (Q) component. Using an IQ modulator the I and Q components of the signal are then modulated on the carrier frequency with a phase shift of 90°.
There also is a trend towards direct-up conversion in the transmitter. Using direct-up conversion, the different channels are directly converted to their corresponding carrier frequencies without the use of an intermediate frequency (IF). Up-conversion is usually done using an analogue modulator, often an IQ modulator. The direct-up architecture has a number of advantages compared to solutions with intermediate frequencies. For example, there are less components needed and the performance of DA-converters (digital-to-analogue converters) is best near DC (direct current).
Analogue modulators generate unwanted image signals in the multi-carrier signals due to imbalances. In case of IQ modulators the imbalances comprise gain differences in between the I path and the Q path and a phase deviation between the I path and the Q path, which is not exactly 90°. In the case of direct-up conversion, the unwanted images fall into the signal band itself and in general cannot be eliminated by filtering. Furthermore, said imbalances depend on the frequency of the signal, which makes image compensation for multi-carrier signals covering a wide frequency range difficult.
According to standard requirements, all transmitted signals have to fulfill certain spectral masks and the images have to be limited below certain levels. High requirements are for example given in the GSM 900 standard for GSM transmission in the 900 MHz band. Good image suppression is therefore needed for the realization of multi-carrier transmitters for the GSM 900 band.